At Exclaimer, we see a lot of email signatures. We see what a lot of companies get right and what they get wrong.
DO make it simple
Keep the layout as simple as possible! If you try something over-the-top or ambitious, be prepared to back it up with lots of testing.
If you have a decent way to test how it will look in different email clients (Outlook, Apple Mail, Gmail), then be sure that it’s solid enough to stay well-presented in various circumstances.